Raman and electronic Raman spectra of lanthanide ions in elpasolite lattices

Raman spectra have been recorded from 300–10 K for lanthanide hexachloroelpasolites, ${\mathrm{Cs}}_2$ ${\mathrm{NaLCl}}_6$. Electronic Raman-scattering intensity ratios have been calculated using Judd-Ofelt-Axe theory and are in reasonable agreement with experimental results, except for those cases where electron-phonon coupling has important consequences. Bands resulting from temperature-dependent electron-phonon coupling phenomena have been observed not only in the Raman spectrum of ${\mathrm{Cs}}_2$ ${\mathrm{NaYbCl}}_6$ but also in that of ${\mathrm{Cs}}_2$ ${\mathrm{NaTmCl}}_6$, and the interaction mechanism is discussed. The coupling element between the coupled a${\mathrm{\Gamma }}_5$∼${\mathrm{\Gamma }}_1$+${\nu }_5$ phonon-electron states of ${}_{}{}^3{}_{}{}^{}\mathrm{H}_6^{}{}_{}{}^{}$ in ${\mathrm{Cs}}_2$ ${\mathrm{NaTmCl}}_6$ is estimated to be 20 ${\mathrm{cm}}^{\mathrm{-}1}$, and the corrected wave functions for these coupled states enable a consistent reinterpretation to be made of the optical spectra of ${\mathrm{TmCl}}_6^{3\mathrm{-}}$. Excited-state electronic Raman scattering has been observed in the 120-K Raman spectrum of ${\mathrm{Cs}}_2$ ${\mathrm{NaTmCl}}_6$. Using fixed wavelength argon-ion laser lines, there is evidence for a small resonance enhancement of electronic Raman-scattering intensity, and the unusual mechanism for this is discussed. The mixed crystal system ${\mathrm{Cs}}_2$ ${\mathrm{NaGd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Yb}}_{\mathrm{x}}$ ${\mathrm{Cl}}_6$ exhibits unimodal behavior for the ${\nu }_1$ and ${\nu }_5$ vibrations, and the more complex behavior of features in the neighborhood of ${\nu }_2$ is described. The vibrational energies of the gerade internal ${\mathrm{LCl}}_6^{3\mathrm{-}}$ modes (determined from Raman spectra) vary linearly with atomic number of L, except where electron-phonon coupling effects occur.